This invention generally relates to an eddy current probe system and method, and is specifically concerned with an improved probe system having a plurality of concentrically-disposed coils for simultaneously detecting different types of flaws or disconinuities at different depths within a wall of, for example, an Inconel.RTM. tube.
Eddy current probes for inspecting the walls of metallic conduits are known in the prior art. Such probes are particularly useful in inspecting the Inconel tubes used as heat exchangers in nuclear steam generators for flaws caused by corrosion or fretting. Generally, these eddy current probes comprise a coil mounted in a probe head that is slidably movable within the interior of the tube being inspected, and electrically connected to a current generator which conducts an alternating current through the coil as it is moved. The current generator is typically capable of generating alternating currents having frequencies of between 10 kHz and 1 MHz. An impedance detecting circuit, which may take the form of an inductive bridge. is also connected across the leads of the coil. In operation, the alternating current conducted through the coil excites it into generating a pulsating magnetic field whose magnitude and polarity changes in accordance with the frequency of the current. When the coil of the probe is positioned in the vicinity of an electrically conductive wall, the changing magnetic flux emanating from the coil induces eddy currents in a portion of the wall. The particular voltage, amperage and direction of the eddy currents produced are dependent in part upon the specific impedance of the portion of the wall that conducts the eddy current. Because the direction of flow of the eddy currents generated by the coil is opposite to the current flowing through the probe coil, the magnetic field created by the eddy currents creates an impedance in the probe coil. The impedance experienced by the eddy currents is in turn dependent upon the resistance these currents encounter as they circulate through the wall. Since flaws in the metal wall (such as cracks, pits, or regions of local thinning) create regions of higher resistances at the flaw locations, eddy current probes may be used to locate flaws by constantly monitoring the impedances of the coils as the probe coils are moved along the walls of the tube. Sharp changes in impedance over localized areas would indicate the existence of cracks or pits or other relatively small-area flaws, whereas gradual changes in impedance over a broad region of the conduit might indicate large-area flaws such as a grain change in the metal, an area of material creep, or a thinned wall region.
Typically, such prior art eddy current probes utilize either a single bobbin type coil whose axis of rotation is parallel with the longitudinal axis of the conduit being inspected (and which is operated in an "absolute" mode), or a pair of bobbin coils having the same radius which are spaced apart from one another (which are operated in a "differential" mode). In either case, the probe head containing either the single or the double coil is moved within the interior of the tube along its longitudinal axis. More recently, "pancake-type" probe coil configurations have come in to use. In such configurations, the axis of rotation of the windings of a relatively flat coil is disposed along the radius of the conduit, and the coil is used to scan the inner wall of the conduit by moving it both radially and longitudinally, thereby imparting a helical motion to the coil. Such pancake-type coils are capable of more accurately locating the precise point where some types of flaws reside in the conduit wall. Both configurations have been successfully used in hostile environments where direct inspection of tubing is impossible, such as the walls of the approximately 40 miles of Inconel tubes used as heat exchangers in nuclear steam generators.
While such probes are capable of performing satisfactory inspections of such heat exchanger tubes, the applicants have noted a number of problems associated with these probes which, up to now, has limited their usefulness. For example, since the depth of penetration of a particular pulsating magnetic field is dependent upon its frequency, it is difficult or impossible for a coil conducting an alternating current of a fixed frequency to simultaneously and reliably resolve flaws at all depths of the conduit wall. Secondly, while a small-diametered coil is best able to accurately pinpoint the location of a particular flaw, such a coil is incapable of transmitting pulsating magnetic fields having frequencies low enough to completely penetrate the conduit walls if the coil diameter is made too small. The applicants have observed that these constraints often necessitate multiple scans of the tube wall with different diametered coils operating at different frequencies if all of the flaws therein are to be accurately and reliably located. However, the multiple scanning of a particular tube wall with different probe coils operating at different current frequencies greatly protracts the time necessary for the testing which in turn results in increased down-time for the steam generator being inspected. As the typical revenue losses associated with such generator down-time often exceeds $100,000.00 a day, the expenses associated with the necessary multiple scans are very substantial Moreover, such multiple scanning also increases the time that the inspecting personnel are exposed to potentially hazardous radiation, which adds even more to the cost of the eddy current testing.
In view of the foregoing, the applicants have concluded that there is a need for an eddy current probe system which is capable of accurately and reliably plotting all types of flaws at all different depths within the walls of a small diametered metallic tube with only a single rapid scan. Ideally, such an eddy current probe system would provide better resolution than any prior art probe system or combination of any such systems. Finally, the eddy current probe system should be extremely versatile, and capable of instantaneously adjusting its pulsating magnetic fields for maximum coupling with the flaw areas detected so as to afford maximum resolution under a broad range of conditions.